Despite the advances in the field of composite technology and its use in restorative dentistry, there has been no reported systematic study of the role of filler parameters on the properties of composite materials. Among these parameters arefiller material composition, filler volume fraction and average filler particle size. There is a need to understand the role of each of these parameters on the properties of composites under conditions of known particle size distribution and fixed resin matrix composition. This proposal seeks to determine the relationship between variations in filler material composition, average filler particle size and filler volume fractions and the corresponding changes in several properties of composite resins. For this purpose, filler material composition, filler volume fractions and average filler particle size will be varied in a series of experimental composite formulations. A 4x5x5 factorial design will be used for the layout. Four filler compositions to be studied are Barium borosilicate; strontium borosilicate; Borosilicate and quartz. Five different average particles sizes in the size range 1 - 10 um and five different volume fractions in the range 0.4 less than or equal to Vf less than or equal to 0.75 will be used in the study. The following properties will be evaluated: (1) compressive strengths, (2) Diametral tensile strenght, (3) Barcol hardness, (4) Depth of cure of visible light cured formulation, (5) wear, (6) polishability and (7) watersorption. One chemical cure and one light cure resin composition will be used as the matrix phase. Analysis of the data will involve testing of the null hypothesis by multivariate analysis of variance (MANOVA) followed by regression analysis if the null hypotheses are rejected. Microstructural analysis of optimized and other selected formulations will be focused o the presence of voids and particle- matrix interfacial integrity.